1. Field of the Invention
The present invention relates to nitride-based semiconductor light-emitting devices.
2. Description of the Background Art
Nitride-based semiconductor is used as a material for high-brightness blue LED (light-emitting diode) and pure green LED that have already been employed for full-color displays for example. Moreover, nitride-based semiconductor is being developed for use in a semiconductor laser as a short-wavelength light source, for the purpose of increasing the storage capacity of an optical disk. These devices have been formed on an insulating sapphire substrate, and thus it is impossible to direct electric current from the rear side of the substrate. Accordingly, p-type and n-type electrodes are formed on the front side, which requires a complicated process for fabricating the electrodes.
According to recently presented reports about fabrication of semiconductor lasers and LEDs, a mask member is partially formed on a sapphire substrate for growth thereon (selective lateral growth), and then a high-quality n-GaN bulk crystal is produced by hydride vapor phase growth. The bulk crystal is used as a substrate having its rear side from which electric current can be directed.
FIG. 6 shows a cross section of a semiconductor light-emitting device including, as a light-emitting element, an LED chip formed of nitride-based compound semiconductor.
The LED chip includes an n-GaN substrate 601 with (0001) or c-plane on which a nitride-based compound semiconductor multilayer film 602 is formed. Nitride-based compound semiconductor multilayer film 602 includes a buffer layer, an n-GaN cladding layer, a multiple quantum well active layer, a p-AlGaN protective layer, and a p-GaN contact layer that are successively deposited. On the uppermost surface of nitride-based compound semiconductor multilayer film 602, a p-type translucent electrode 603 and a P pad electrode 604 are formed. On the rear surface of n-GaN substrate 601, an n-type electrode 605 formed of stacked Ti and Al is formed. The LED chip is 350 μm×350 μm in size.
The LED chip is attached firmly onto a cup 608 of a leadframe 607 with Ag paste 606, and the LED chip and leadframe 607 are electrically connected by P pad electrode 604 and a wire 609 and by n-type electrode 605 and Ag paste 606. Leadframe 607 with the LED chip mounted thereon and wire 609 are encapsulated in mold resin 610 to form a semiconductor light-emitting device. The semiconductor light-emitting device has characteristics that the luminance is 3.0 Cd and the operating voltage is 3.5 V at room temperature under a current of 20 mA. It is noted that the luminance and operating voltage are hereinafter represented as values measured at room temperature under a current of 20 mA.
100 semiconductor light-emitting devices as described above are prepared on which an energization test is conducted under atmospheric pressure with a constant drive current of 30 mA. After 5000 hours from the start of the test, the operating voltage of the semiconductor light-emitting devices is measured. The rate of change of the measured operating voltage relative to the operating voltage before the start of the test is +20%. Here, the rate of change is defined as a value determined by dividing, a difference between the measured operating voltage and the operating voltage before the start of the test (hereinafter pre-test voltage), by the pre-test voltage and multiplying the resultant quotient by 100. The rate of change is indicated by “+” when the voltage increases while that is indicated by “−” when the voltage decreases. The lamps (light-emitting devices) undergoing the test are disassembled and accordingly analyzed. It is found that the n-type electrode of the chip attached to the cup is on the point of peeling off. A further test and more detailed analysis prove that this peeling of the n-type electrode is caused by thermal strain from the cup surface.